Gas turbines are employed in many fields for driving generators or operational machines. In this case, the energy content of a fuel is used to generate a rotational motion of a turbine shaft. For this purpose, the fuel is burnt in a combustion chamber, compressed air being supplied by an air compressor. The working medium, which is generated in the combustion chamber by the combustion of the fuel and which is at high pressure and high temperature, is then guided via a turbine unit connected downstream of the combustion chamber and expands, while performing work, in this turbine unit.
In order to generate the rotational motion of the turbine shaft, a number of turbine blades are arranged on the turbine shaft. The blades are usually combined into blade groups or blade rows and drive the turbine shaft by use of a transfer of momentum from the flow medium. In order to guide the flow medium within the turbine unit, furthermore, guide vane rows connected to the turbine casing are usually arranged between adjacent rows of rotor blades. In this arrangement, the turbine blades/vanes, in particular the guide vanes, usually have a profiled blade/vane aerofoil extending along a turbine blade/vane center line for the appropriate guidance of the working medium. In order to fasten the turbine blade/vane to the respective support body, a platform extending transverse to the blade/vane aerofoil and embodied as an engagement base is formed at the end of the blade/vane aerofoil.
In order to achieve a particularly favorable efficiency, such gas turbines are, for thermodynamic reasons, usually designed for particularly high outlet temperatures—approximately 1200° C. to approximately 1300° C.—of the working medium flowing out of the combustion chamber and into the turbine unit. With such high temperatures, the components of the gas turbine, in particular the turbine blades/vanes, are exposed to comparatively high thermal loadings. In order to ensure a high degree of reliability and a long life of the respective components, even under such operating conditions, the components affected are usually configured in such a way that they can be cooled.
For this purpose, the turbine blades/vanes are usually embodied as so-called hollow profiles in modern gas turbines. The profiled blade/vane aerofoil has cavities, also designated as blade core, in its interior region for this purpose, in which a coolant can be conducted within these cavities.
Admission of the coolant to the thermally, particularly loaded regions of the respective blade/vane aerofoil is made possible by the coolant ducts formed in this way. In this arrangement, a particularly favorable cooling effect, and therefore a particularly high level of operational reliability, can be achieved by the coolant ducts taking up a comparatively large spatial region within the respective blade/vane aerofoil and by the coolant being conducted as close as possible to the respective surface exposed to the hot gas. In order to ensure an adequate mechanical strength and load-carrying capability in such a configuration, on the other hand, the respective turbine blade/vane can have flow passing through it in a plurality of ducts; such a plurality of cooling ducts, which can be exposed to coolant and are respectively separated from one another by comparatively thin separating walls, is then provided within the blade/vane profile.
Such turbine blades/vanes are usually manufactured by casting. For this purpose, a casting mold, whose contour is matched to the desired blade/vane profile, has wax poured into it in a first casting step. In order to manufacture the flow ducts for the coolant, so-called core elements, in ceramic material for example, are arranged in the casting mold during the casting. After the casting procedure has taken place, these are removed from the wax model for the blade/vane body so that the cavities desired for the coolant ducts appear. The wax model obtained in the first casting step is subsequently provided with a ceramic coating by means of repeated immersion.
As soon as this ceramic coating has a sufficient thickness, if required after a plurality of immersion procedures, the wax model provided with the ceramic coating is burnt out, in which procedure the ceramic is strengthened and the wax is burnt out. By this, a ceramic casting mold for the blade/vane appears in which the core elements for cooling ducts are inter alia also included. In a second casting step, this ceramic casting mold has blade material poured into it. In order to manufacture the wax model, and in particular its blade/vane aerofoil and the structural parts formed on it, such as the platform or an engagement base, appropriately shaped shell elements or slides are arranged in the casting mold for the first casting step. This is done in such a way that, during the casting procedure, a cavity corresponding to the blade/vane shape to be manufactured remains for accepting the wax.